Filters are extensively used in communication devices, particularly in radio receivers, to provide selectivity for the received signals. A number of factors, including the type and number of resonators in the filter topology, determine the selectivity of a filter. Depending on the application, the filter topology may include any number of quarter-wave resonators, half-wave resonators, or a combination of them.
In order to form a particular filter topology, transmission line filters provide an attractive alternative to filters which utilize discrete components. Conventional stripline or microstrip resonators typically utilize a substrate which is made of ceramic or another dielectric material. For microstrip construction, a conductive runner is formed on one side of the substrate with a ground plane on the other side. The stripline configuration utilizes two such structures with ground planes on the outside and conductive runners therebetween. The resonant frequency of the resonators is determined by such factors as the dielectric constant of the substrate, the thickness of the substrate, and the length and the width of the conductive runner. An inverse relationship exists between the size of the transmission line structure and the resonant frequency of the resonator. That is, for lower resonant frequencies, a substantially longer transmission line structure is needed and vice versa.
A quarter-wave resonator may be produced by providing a ground path at one end of the conductive runner. A half-wave (or a full-wave) resonator may be produced by either grounding both ends of the conductive runner or by providing openings at both ends. The transmission line filter is produced by forming a particular resonator configuration, including different types of resonators, (that is, half-wave or quarter-wave), on the dielectric substrate to create the desired filter topology.
Generally, transmission line filters utilize a number of interdigitated quarter-wave length resonators to provide the desired passband for a specified selectivity. However, the specified selectivity may also be achieved by tuning a transmission zero produced by capacitive coupling of the resonators which are formed in a comb-line arrangement on the filter substrate. Conventionally, the transmission zero frequency is tuned by controlling the capacitive coupling between the resonators by means of varactors which have one terminal coupled to the open ends of each of the resonators and voltage at their other terminals which are coupled to each other. In this arrangement, the DC ground path for the varactors are provided through the grounded end of the quarter-wave resonators. This arrangement, however, requires two varactors and a larger transmission line structure, especially when lower frequency pass band filters in UHF and VHF bands are needed.
In my pending U.S. patent application, Ser. No. 07/676,023 filed on Mar. 27, 1991, and assigned to the Motorola, Inc., the assignee of the present invention, which is hereby incorporated by reference, I disclosed a transmission line structure having two resonators disposed on a substrate. At least one of the two resonators comprises an open ended half-wave resonator. The other resonator has at least one open end. A varactor is disposed between the open ends of the two resonators and controls capacitive coupling therebetween for tuning the transmission zero. The control voltage terminal is positioned along the length of the open ended resonator at a point where a zero potential exists at the resonant frequency. A control voltage applied at this point varies the capacitance of the varactor and may be used to tune the transmission zero. However, because capacitive coupling of the two resonators is varied for tuning, the filter passband is not constant.
Therefore, it is desired to provide a simple, highly-selective filter which may be tuned without affecting its pass band.